Descriptions

Helium isotopes are a robust geochemical tracer of a primordial mantle component in hot spot volcanism. The high ³He/⁴He (up to 35 RA, where RA is the atmospheric ³He/⁴He ratio of 1.39 X 10¯⁶) of some Hawaiian Island volcanism is perhaps the classic example. New results for picrites and basalts from the Hawaiian-Emperor seamount chain indicate that the hot spot has produced high ³He/⁴He lavas for at least the last 76 million years. Picrites erupted at 76 Ma have ³He/⁴He (10-14 RA), which is at the lower end of the range for the Hawaiian Islands but still above the range of modern mid-ocean ridge basalt (MORB; 6-10 RA). This was at a time when hot spot volcanism was occurring on thin lithosphere close to a spreading ridge and producing lava compositions otherwise nearly indistinguishable from MORB. After the hot spot and spreading center diverged during the Late Cretaceous, the hot spot produced lavas with significantly higher ³He/⁴He (up to 24 RA). Although ³He/⁴He ratios stabilized at relatively high values by 65 Ma, other chemical characteristics such as La/Yb and ⁸⁷Sr/⁸⁶Sr did not reach and stabilize at Hawaiian-Island-like values until ~45 Ma. Our limited ³He/⁴He record for the Hawaiian hot spot shows a poor correlation with plume flux estimates (calculated from bathymetry and residual gravity anomalies [Van Ark and Lin, 2004]). If ³He is a proxy for the quantity of primordial mantle material within the plume, then the lack of correlation between ³He/⁴He and calculated plume flux suggests that variation in primordial mantle flux is not the primary factor controlling total plume flux.